Accurate alignment of the wheels in an automobile or other vehicle is important for proper handling of the vehicle and proper tire wear. Correct wheel alignment also decreases fuel consumption and improves vehicle safety. Typical wheel alignment systems measure parameters such as camber, caster, steering axis inclination and toe.
There are many different types of devices and systems that have been used to perform one or more of the wheel alignment measurements. Although the sensing scheme used may vary, many modern alignment systems utilize sensor heads or targets mounted to the vehicle wheels, and a computer, to process sensed signals to determine the alignment parameters. In any such system, accurate measurements depend on accurate calibration of the alignment system components. A manufacturer may calibrate the wheel alignment system, as part of the manufacturing and/or installation process, and the system may be re-calibrated from time to time to compensate for wear and tear. A variety of devices and techniques have been used to calibrate the different types of wheel alignment systems.
It is desirable to certify that measurements taken using a calibrated system satisfy industry standards, for example, the relevant standard set by the International Organization for Standardization (ISO). Stated another way, to show that a vehicle has been aligned in accord with a standard requires that alignment of the vehicle wheels has been performed using an alignment system that has been tested and certified for compliance with the standard. To show that an alignment system qualifies under the standard, a need exists to certify or verify that the accuracy of the calibration of the wheel alignment equipment falls within the acceptable levels specified in the relevant standard.
In the past, one approach, particularly as used by vehicle original equipment manufacturers (OEMs), has been to take measurements of system performance using a rolling master that had been qualified in advance to correspond to appropriate national standards. The rolling master was a robust device, approximately the size of a vehicle, and it was qualified using a theodolite system or a laser tracker to have precisely known wheel alignment parameters. The manufacturer's technicians would operate the wheel alignment system that was to be certified, so as to measure wheel plane angles and positions in space on the master. The alignment system would convert the measured parameters to vehicle coordinates and would process those values to derive alignment parameters for the master, such as its toe and camber. Positional information could also be expressed as wheelbase, track width, setbacks, lateral offsets and the like. The various derived values for alignment parameters and/or positional information were then compared to the known values for the qualified rolling master, to determine measurements of the system accuracy with respect to each value determined by the alignment system. If the accuracy of the system under test were within certain limits with regard to acceptable accuracy (e.g. in compliance with the relevant standard), then the manufacturer would certify the accuracy of the system and thus of the alignment of its vehicles performed using that system. Although this approach, using the rolling master was adequate for OEMs or the like, it was not readily adaptable to application in common auto shops. For example, the rolling master was relatively difficult to transport, and it is simply not practical to have such a master at every tire store or auto dealership that does wheel alignments.
Another method used to show good calibration of an aligner with wheel-mounted measurement instruments was to take normal measurements on one or two calibration bars and determine if total toe and camber values were zero. However, this technique did not provide traceability of accuracy, to allow certification. This check of calibration did not use verification means separate from the primary calibration means and did not provide individual toe confirmation.
Hence a need exists for an enhanced technique for providing verification or certification of calibration accuracy of a wheel alignment system, which is readily adaptable to periodic certification of aligners that may be installed in many different shop locations. Hence, any solution should utilize portable or collapsible certification equipment so as to be relatively easy to transport to or store at shop sites. The certification equipment should be easy to assemble and use, and should itself be relatively easy to qualify as to the accuracy of its characteristics.